Analog cellular telephone systems have been established for many years. The typical analog single carrier per caller (SCPC) system uses frequency modulated (FM), 25 kHz wide radio channels whose carriers are separated by 30 kHz. Unfortunately, increased use of existing analog cellular systems has resulted in corresponding increases of call blocking--particularly in congested urban areas. Recently, it has become advantageous to increase the capacity of existing frequency spectrum allotted by the FCC for cellular communications using time division multiple access (TDMA) techniques. TDMA is a digital communication method that permits plural voice channels to be transmitted as a series of digital codes interleaved on a single carrier.
In a TDMA system, a single RF carrier is divided into logical time frames of a predefined length. Each frame is further divided into plural time slots, e.g. six, with each time slot potentially representing a separate channel. Most time slots are used to transmit subscriber voice or data information. Remaining time slots a Fe usually provided to transmit control information. Thus, if even just two subscribers can use the same carrier frequency, then the cellular system capacity is essentially doubled.
Because of the obvious advantages of digital communication techniques like TDMA, there has been a move to abandon existing analog cellular systems in favor of more powerful digital cellular systems. However, there is already a significant investment in the existing analog SCPC infrastructure. As a result, a new standard has been adopted which supports both the existing analog SCPC format and the digital TDMA format. The dual-mode standard, known as the IS-54, requires that dual-mode mobile telephones be operable in either an analog or a digital communications mode.
The IS-54 standard for analog communication uses frequency modulation (FM). Because FM is a constant envelope modulation scheme, the modulated information not sensitive to variations of carrier amplitude and therefore may be amplified without loss or distortion information by a nonlinear amplifier operating in saturation. For digital communications the IS-54 standard requires a modulation scheme known as .pi./4-shifted (Differentially encoded Quadrature Phase Shift Keying) which more efficiently uses the allocated frequency spectrum. However, .pi./4-shifted DQPSK performs some amplitude modulation of the transmitted signal, therefore, distortion of the signal amplitude cause,. nonlinear amplification cannot be tolerated. Accordingly, in digital TDMA systems, linear amplifiers must be used.
Linear operation infers that for every decibel change in drive power, the output power changes exactly the decibel amount. Moreover, linear operation faithfully reproduces the drive signal without distortion or unwanted spurious products and occurs as long as operation is restrained below the saturation point of the amplifier.
Amplifiers are often classified according to the conditions under which the transistor operates, i.e., according to the portion of the AC signal voltage cycle during which the transistor output current flows as controlled by the bias on the base or gate of the transistor. The four classes of amplifier operation a Fe generally recognized as A, B, AB, and C. In class A operation, the transistor operating point is biased near the midpoint of the linear portion of the transistor characteristic curve. The RF signal input causes the transistor output to vary above and below that operating point. The transistor conducts throughout the entire input signal cycle with the transistor output being directly proportional to the input signal. In other words, the transistor operates within the linear portion of its characteristic curve to provide linear amplification. The principal characteristics of class A amplifiers are minimum distortion, and relatively low efficiency, e.g. 20 to 25%.
In class B operation, the transistor is biased near its cutoff value. The RF input signal drives the transistor into cutoff for approximately half of a cycle. Thus, the transistor conducts for about 180.degree. of the input signal cycle and is cutoff during the other 180.degree.. In class AB operation, the transistor conducts for greater than 180.degree. but less than 360.degree.. Such amplifiers are characterized by medium efficiency, e.g. 40-60%. In class C operation, the transistor is biased into cutoff so that the transistor conducts appreciably less than 180.degree.. Thus, the transistor remains turned off for most o f each input signal cycle, and the transistor output is a series of pulses. Although class C amplifiers distort the input signal, they have a high efficiency, e.g. 70 to 80%.
Because of their low distortion and high efficiency, class AB amplifiers are best suited for the .pi./4- shifted DQPSK modulation used in the digital communications mode as described above. Although the operating or Q-points of class AB type amplifiers are set for linear operation, at some saturation point their operation becomes nonlinear. That saturation point may be achieved by driving the amplifier with an input voltage signal having a magnitude sufficient to "clip" the output voltage. As a result, the output waveform resembles, depending on the degree of clipping, a square wave rather than a sinusoidal wave.. Clipping occurs when the peak voltage swing of the output signal attempts to exceed the supply voltage to the amplifier.
A square wave output provides more efficient amplifier operation because the transition time between voltage maximum/current minimum and voltage minimum/current maximum conditions is minimized. Since power is dissipated only during that transition time, the amplifier operates more efficiently when saturated. Changing the supply voltage to a linear amplifier changes the saturation point. Thus, by controlling the supply voltage, both linear and nonlinear operation can be achieved.
The dual-mode IS-54 standard requires that the average power output over a standard time period in the digital mode be equal to the average power output in the analog mode. Referring to FIG. 1(a)-1(b), it is evident that the peak power output in the analog mode is a constant power value P.sub.O. In contrast, digital transmissions occur as amplitude varying bursts during allotted time slots as shown in FIG. 1(b). Therefore, the peak power required of a power amplifier in a mobile telephone in the digital mode necessarily must be larger in the digital mode than in the analog mode to achieve the same average power output.
While a single unsaturated linear amplifier could be used in both digital and analog modes, efficiency power are unacceptably sacrificed when operating in the analog mode where the peak power requirement is significantly lower than-that required in the digital mode. Moreover, in the analog mode, the battery life of the dual-mode mobile is unnecessarily shortened. Battery life in mobile telephones corresponds directly to talk time, and talk time is a significant factor in consumer differentiation of competing products.
Two separate RF power amplifiers could be used--one for the analog mode and one for the digital mode--to maximize efficiency and therefore battery life. While the use of two separate RF power amplifiers may address the problem of efficiency, this approach significantly increases the cost and size of portable radiotelephones.
What is needed is a means for maximizing the efficiency of a dual-mode portable radiotelephone without significantly increasing the cost and size of that radiotelephone.
The present invention provides a single power amplifier for a portable, dual-mode radiotelephone with high efficiency amplification when operating in the analog mode and linear amplification when operating in the digital mode. In the dual-mode radiotelephone, control of the power provided from a power supply network permits the single power amplifier to operate in two different modes: a saturated mode to permit high efficiency, low power consumption operation in the analog mode and an unsaturated mode to permit linear operation in the digital mode. Specifically, the power supply network permits high efficiency nonlinear amplification for constant RF envelope modulated signals and linear amplification of input signals which have been modulated using modulation techniques that vary, at least to some extent, the amplitude of the RF carrier.
The power supply according to the present invention for providing different voltage levels to an RF power amplifier in a mobile radio telephone specifically may include a battery; complementary transistors oppositely biased by a drive signal, where the drive signal has two logic states; and a capacitor connected between the transistors such that the capacitor is alternately placed in parallel with the battery when the drive signal is at a first logic state and in series with the battery when the drive signal is at a second logic state. A diode is connected between the capacitor and the transistors to prevent discharge of the capacitor through the transistors.
A first voltage is generated by the power supply when the drive signal is at the first logic state and a second voltage is generated by the power supply when the drive signal is at the second logic state. The drive signal may be generated using the TDMA frame structure and synchronization signals of the digital portion of the dual-mode radiotelephone to achieve the two different modes of amplifier operation (analog and digital) accurately and inexpensively. In one embodiment, the complementary transistors include an NPN bipolar transistor and a PNP bipolar transistor where the base terminals of the transistors are connected to the drive signal, the emitter of the PNP transistor is connected to the DC voltage source and the anode to the diode and the cathode to the diode is connected to the collectors of both the transistors via a capacitor. Alternatively, complementary N-type and P-type field effect transistors (FET's) can be used in place of bipolar transistors, often with lower losses.
The power supply network according to the present invention may also employ plural dc voltage sources for providing different voltage levels of dc power to drive a RF power amplifier in linear and nonlinear modes depending on the voltage level supplied. Plural switches connected to the dc voltage sources are actuable in accordance with the drive signal to provide different dc power voltage levels. A capacitor is connected by the switches such that the capacitor is alternately placed in parallel with the plural dc voltage sources and in series with one of the plural dc voltage sources depending on the actuation states of the switches. A diode is connected to the capacitor to prevent discharge through the switches.